The Nike Free Run broke the constraints of what a traditional running shoe is supposed to look or feel like. Originally engineered in 2004 by Tobie Hatfield and Eric Avar, this game-changer was inspired by Stanford athletes that were found training barefoot on a field. The series has a numbering system that represented the amount of cushioning in each model. The Nike 3.0 Free is the lightest and lowest and the Nike 7.0 Free has the most cushion and support. The Nike 4.0 Free and Nike 5.0 Free is a nice compromise of lightweight comfort and supreme flexibility. Today, Nike Frees are worn casually just as much as they are worn for working out, thanks to their lightweight, flexible design and sleek aesthetic.